As known in the conventional art, a motor outputs power by energizing a coil to rotate a rotor. When energizing the coil, a copper loss is caused inevitably by Joule heating, and a core loss is also caused inevitably by a change in the magnetic field resulting from rotation of the rotor. As a result, heat is produced in the motor thereby raising the temperature thereof. If the temperature of the motor is thus raised, an output, durability etc. of the motor are degraded by a degaussing or the like. Especially, the motor for driving an automobile is used under various conditions. For example, the motor used in the automobile is sometimes rotated at a low speed while generating a high torque, but sometimes rotated at a high speed while generating a low torque. Therefore, the motor used in the automobile is easily to be heated. However, since the motor of this kind is housed tightly in the limited space together with other members, the motor is difficult to be cooled naturally in the automobile.
For example, techniques for compulsory cooing an end portion of a stator coil are disclosed in an International patent Laid-Open WO/2004/019468, Japanese Patent Laid-Open No. 2009-118667, and Japanese Patent Laid-Open No. 10-290543. Specifically, International patent Laid-Open WO/2004/019468 discloses a motor comprising a coil end cover covering an end portion of the stator coil, and a supply pipe for feeding cooling oil into the coil end cover. According to the teachings of International patent Laid-Open WO/2004/019468, a cooling oil inlet is formed at a lower end of the coil end cover, and an oil pump driven by an engine is connected with the oil inlet. In addition, a cooling oil discharge outlet is formed at an upper end of the coil end cover, and an oil pan is connected with the discharge outlet. According to the motor taught by International patent Laid-Open WO/2004/019468, therefore, the cooling oil is supplied to the coil end cover by driving the oil pump by the engine, and the heat of the coil is drawn by the cooling oil flowing through the pipe while being contacted with the end portion of the coil. Then, the cooling oil is retuned from the discharge outlet to the oil pan, and supplied again to the coil end cover by the oil pump.
Japanese Patent Laid-Open No. 2009-118667 discloses a motor cooling structure in which a coil end cover covering an end portion of a stator coil is arranged between a coil end and a motor case. Therefore, the coil is cooled by supplying cooling liquid to a clearance between any of the coil end cover, the coil end and the motor case. In addition, the motor cooling structure taught by Japanese Patent Laid-Open No. 2009-118667 further comprises micro gaps into which the cooling liquid leaks. Therefore, air acting as a heat insulator is pushed out of the heat transmitting route from the coil end to the motor case by the cooling liquid flowing into the micro gap.
Japanese Patent Laid-Open No. 10-290543 discloses a motor comprising a coil end cover covering a coil end, and a spacer arranged between the coil end cover and a motor housing. According to the teachings of Japanese Patent Laid-Open No. 10-290543, heat is transmitted to the motor housing through the spacer, and the heat thus transmitted to the motor housing is further transmitted to the water in a water jacket to cool a coil.
As disclosed in International patent Laid-Open WO/2004/019468 or Japanese Patent Laid-Open No. 2009-118667, the heat of the stator coil can be drawn by supplying the cooling oil or cooling liquid around the coil end. However, as described, the motor is driven under various conditions. For example, the motor is sometimes driven at a low speed while being required to generate a high torque, but sometimes driven at a high speed while being required to generate a low torque. That is, if the cooling oil or the cooling liquid is always supplied at a constant flow rate, the cooling oil or the cooling liquid may be insufficient under the situation in which the motor is driven at a low speed while generating a high torque, and the temperature of the motor is thereby raised. To the contrary, in case the motor is driven at a high speed while generating a low torque, the cooling oil or the cooling liquid may be supplied in excessive amount and this may cause a power loss. Especially, provided that the vehicle is structured to dive the pump by a driving motor, and to supply the cooling liquid pressurized by the pump around the coil end, in case the vehicle thus structured climbs a hill, for example, the driving motor is driven at a low speed while supplying a large amount of current thereto. In this situation, a rotational speed of the pump is lowered and the flow rate or flow amount of the cooling liquid is thereby reduced, however, a heat generation amount of the motor is increased. Therefore, the motor may not be cooled sufficiently and the temperature of the motor is thereby raised in this situation.